1. Field of the Invention
The present invention generally relates to a method and apparatus for lithography, and more particularly to a method and apparatus for imprint residual layer management.
2. Description of the Related Art
Imprint lithography describes a class of lithographic methods in which a flat mold 10 (e.g., a transparent mold or template) is pressed into a liquid polymer (resist) 11 on a flat substrate 12, as shown in FIG. 1A.
Then, as shown in FIG. 1B, the polymer 11 is cured by exposure to light 13 (e.g., in the case where a transparent mold is used) or heat.
Thereafter, as shown in FIG. 1C, the mold 10 is removed leaving behind an impression of the features of the mold 10 in the polymer 11. In practice, the mold 10 is typically flat with fine depth features etched in its surface. In cases of practical interest, these features can have dimensions that range from many microns to nanometers. The intent is usually to transfer the relief pattern 15 in the polymer resist 11 into the substrate material using an etch process.
FIG. 1D shows the residual layer 16 of the polymer resist 11 after etching to expose the surface of the substrate 12.
It is desirable to press the mold into the resist such that very little resist (e.g., on the order of about 50% of the feature height or less) remains between the unfeatured portions of the mask and the substrate 12. Often as little as 40 nm thickness or less is desirable.
Further, it is essential to the subsequent etch process that this residual layer 16 thickness be extremely uniform. Practically, this is extremely difficult to achieve due to the viscosity of the resist, flatness and flexibility of the mold 10 and substrate 12 and particulate contamination.
This results in the need to use ultra-clean, extremely precise, slow and costly methods to perform lithography at the micron to nanometer scales that are of interest. Considerable cost and effort must be expended to develop systems that reduce and homogenize the residual layer.
Thus, a large challenge in the conventional techniques is achieving a suitable ratio of the thickness of the residual layer to the thickness of the feature, and thus such ratio typically dictates the process window. Practically, obtaining the residual layer extremely thin is difficult to achieve, as mentioned above.
Indeed, one can imagine that the mask is 1 centimeter (or up to many inches) square and that the resist is not very viscous, but has a finite viscosity, pressing the same to 50 nanometers is difficult to perform. That is, as the residual layer becomes thinner and thinner, the viscous shear force increases accordingly and eventually a large amount of force is required (for pressing) to achieve the desired thinness. This force has the tendency to warp the mask and substrate causing further inhomogeneities. Finally, as the residual layer thins, more time is required for it to move out of the way.
Additionally, as mentioned above, uniformity is problematic since if one desires a residual layer thickness of 50 nanometers nominally, one wants 50 nanometer thickness everywhere and this requires that the mask be perfect, and that the surface and resist must be particle-free. Any particulate contamination that is larger than the desired residual thickness will cause local distortions of the mask and substrate. These distortions result in a final print defect that is larger in size than the original particle that caused it.